Alkaline earth thiogallate phosphors

ABSTRACT

A phosphor system consisting of alkaline earth and alkali metal thiogallates activated by europium, lead, or cerium. These phosphors are photoluminescent and cathodoluminescent and are suitable for use in fluorescent lamps or cathode-ray tube screens.

United States Patent Peters 1 Feb. 1, 1972 [54] ALKALINE EARTH THIOGALLATE PHOSPHORS [72] inventor: Thomas E. Peters, Levittown, NY.

[73] Assignee: GTE Laboratories Incorporated [22] Filed: July 1, 1969 [2]] Appl. No.: 838,065

US. Cl. ..252/30l.4 S, 252/301 .4 S

Field 0 Search ..2s2/3o1.4s

Primary Examiner-Robert D. Edmonds [57] ABSTRACT A phosphor system consisting of alkaline earth and alkali metal thiogallates activated by europium, lead, or cerium. These phosphors are photoluminescent and cathodoluminescent and are suitable for use in fluorescent lamps or cathode-ray tube screens.

19 Claims, 1 Drawing Figure ALKALINE EARTH THIOGALLATE PHOSPHORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to fluorescent materials which emit light when exposed to ultraviolet or electron radiation. In particular it relates to the thiogallates of alkaline earth and alkali metal elements as activated by europium, lead, or cerium.

2. Description of the Prior Art Luminescent materials comprising zinc, cadmium, and mercury thiogallates activated by copper are known. Europium, lead, and cerium have been used as activators in a variety of host systems (see Some Aspects of the Luminescence of Solids, Kroger, Elsevier Publishing Co. lnc., 1948).

SUMMARY OF THE INVENTION The phosphor compositions of the present invention consist of activated thiogallates of the alkaline earths, calcium, strontium and barium, and activated thiogallates of the alkali metals, sodium and potassium. The activators of the present invention are europium, lead, and cerium. The compositions of this new phosphor system may be represented by the general formulas wherein R is at least one alkaline earth element selected from the group consisting of calcium, strontium or barium; M is at least one alkali metal selected from the group consisting of sodium or potassium; A is an activator chosen from the group consisting of europium and lead; and w is a value selected to produce luminescence when the phosphor composition is exposed to incident radiation.

The alkaline earth thiogallate phosphors of the present invention are cathodoluminescent and photoluminescent and exhibit a wide range of emission colors varying over the visible spectrum, with individual emission color being dependent upon the particular selection of host and activator material and, in some cases, activator concentration. In the discussion herein of the alkaline earth thiogallate phosphors, all emission characteristics are assumed to be for cathode-ray excitation unless otherwise stated, but it should be noted that each phosphors photoluminescent emission color is similar to its cathodoluminescent emission color.

The divalent europium-activated calcium, strontium, and barium thiogallates exhibit yellow, green, and blue-green emission, respectively. A preferred range of europium activator for these phosphors is w equals 0.0] to 0.08 gram-atom per mole. The emission color of these phosphors does not vary substantially with the europium concentration. However, intermediate colors between yellow and blue-green are obtained by employing more than one alkaline earth in a single europium-activated thiogallate phosphor. For example, the phosphor Sl'0.31 0.uu 2 4 om has a yellow-green emission which lies between the yellow emission of the calcium (only) phosphor and the green emission of the strontium (only) phosphor.

The europium-activated strontium thiogallate phosphor of the present invention is a suitable phosphor for the green component of a color'television picture tube. For example, the phosphor Sl'rwxGazSfiEU g when cathodically excited emits bright green light having an emission peak at about 535 nanometers. This phosphor has color coordinates of x=0.272, y=0.685 as depicted by the point 0, in the chromaticity diagram of FIG. 1. FIG. 1 is a standard C.l.E. chromaticity diagram having color definitive .vand y-coordinates whereby color hue and degree of saturation may be designated. Th e points B and R on the diagram correspond respectively to coordinate positions of blue and red phosphors commonly employed in color television picture tubes. Point B (.\=O.l53, v=0.045 represents a blue emitting silver-activated zinc sulfide phosphor and point R (.\=0.665, y=4).335) represents redemitting europium-activated yttrium vanadate. As is well known, the area of the solid-lined triangle 30 made by joining points 0,, R and B is indicative of the gamut of colors which can be obtained from a color system having constituent phosphors with emissions that correspond to these points.

The point 0: represents the color coordinate of a typical silver-activated zinc-cadmium sulfide (.\'=0.2S3, y=0.552), the green-emitting'phosphor conventionallyused in color TV picture. The area of the dashed-line triangle 3| indicates that the gamut of colors available from the color system employing the zinc-cadmium sulfide phosphor is substantially smaller than the gamut of colors available from the same color system in which the disclosed thiogallate phosphor is employed. The advantageous emission color of the europium-activated strontium thiogallate for use in color television tubes will thus be readilyappreciated.

The lead-activated alkaline earth thiogallates, unlike their europium-activated counterparts, tend to significantly change emission color as the amount of the activator is changed. For example, the phosphor Ca Gag Pb appears white and has an emission peak at about 510 nanometers whereas the phosphor Ca GaZS,:Pb appears yellow-orange in color and has an emission peak at about 530 nanometers. This is typical of the lead-activated alkaline earth thiogallates which tend to shift towards higher wavelength emissions as the amount of activator is increased.

When trivalent cerium is used to activate an alkaline earth thiogallate, the trivalent cerium cannot simply enter the crystal lattice in place of the divalent alkaline earth element. If no charge compensation is employed, it is believed that the resulting crystal lattices will have structural vacancies" (a condition known as "vacancy compensation). This vacancy-compensated" phosphor can be represented by the notation R, Ga S,:Ce wherein the empty parenthesis stands for the indicated vacancy. if desired, the cerium-activated alkaline earth thiogallate phosphors of this invention can be charge compensated with either a monovalent or a divalent element. When a monovalent element, for example, sodium, is used, a phosphor composition of the form R, ,,Na,,. Ga,S,:Ce,,. is produced. The preferred range of activator for this sodium-compensated phosphor is w equals 0.0] to 0.12 gram-atom per mole. Potassium can also be used for monovalent charge compensation. When a divalent element, for example, zinc, is used it is believed that the gallium sites are affected and the composition R ,,Zn,,.Ga ,,.S :Ce, is formed.

The cerium-activated alkaline earth thiogallate phosphors of the present invention have a relatively short persistence; i.e., their luminescence persists for only a very short period of time (of the order of a microsecond) after an exciting stimulus is removed. This phenomena is believed to involve an allowed D-- F transition of the cerium ion. Short-persistence is one important requirement for phosphors used in a short-persistence cathode-ray tube'known as a flying-spot scanner" tube. A flying-spot scanner tube which employs the ceriumactivated phosphors of the present invention is disclosed in my copending U.S. Pat. application Ser. No. l25,6ll filed Mar.

l8, 1971, which is a continuation-in-part of my U.S. Pat. application Ser. No. 838,170 filed July 1, I969 and assigned to the same assignee as the present application.

The alkali metal thiogallate phosphors of the present invention exhibit a variety of emission colors when excited by ultraviolet or cathodic radiation. These phosphors are generally brightest under ultraviolet excitation and, accordingly, are useful in such applications as color correction and photocopymg.

Europium-activated potassium thiogallate exhibits blue emission when irradiated with either ultraviolet or cathode rays, while the europium-activated sodium thiogallate has a yellow emission under these same modes of excitation. lntermediate emission colors are obtained when both sodium and potassium are incorporated in the phosphor. For example, the P P Q !JBN Q-fi r has a b l h' emission when excited by either'tiltravioletor cathodic radiation. The preferred range of activator for the europium-activated alkali metal thiogallate phosphors is w equals 0.00125 to 0.08 gram-atom per mole.

The lead-activated alkali metal thiogallates are moderately strong orange-emitting photoluminescent phosphors but emit weakly under cathode-ray excitation. These phosphors have very broad emission bands with peaks that shift to longer wavelengths as the activator concentration is increased.

The cerium-activated alkali metal thiogallates emit blue light when excited by either photoexcitation or cathode rays. Both the sodium and potassium phosphors have short-per- EXAMPLE l4 Europium-activated alkaline earth thiogallate phosphors were prepared by the method of Examples l-3 except that combinations of alkaline-earth elements were included in h ider bl less' h i r i e i' zafli sdz i'f zzzha t n than ihe e jium-zctivated :t r gn each phosphor In a first sample 0'886 gram of strontium y tide and 0.866 gram of calcium sulfide were blended with 4.7l ttum-thiogallate.

grams of gallium sesquisulfide and OJ 1 gram of europium sul- EXAMPLES I42 fide. The resultant phosphor sr ca qmsstliu was excited by ultraviolet (MPMV) and cathodic radiation and The alkaline earth thiogallate compositions shown in Table found to exhibit yellow-green emission peaking at about 547 I were made by dry-blending gallium sesquisulfide with the nanometers under both modes of excitation. In a second samsulfides of an alkaline earth element and an activator element. ple 2.03 grams of barium sulfide were substituted for the calci- The europium-activated and lead-activated compositions um sulfide ofthe first sample. The resultant phosphor SnmBB were made by firing the blend in a hydrogen sulfide (H 8) at- .,.,Ga-,S.,:Eu0.".-| emitted green radiation peaking at about 5 l5 mosphere for about 2 hours at a temperature of approximately nanometers when excited byeither cathodic or ultraviolet l,000 C. and then allowing the blend to cool to room temradiation. perature in a nitrogen atmosphere. The cerium-activated compositions were made by firing the blend in an H 8 atmosphere 2O EXAMPLES for f hours at a p of pp t y Cerium-activated charge-compensated alkaline earth anwmg f cool to mom l p l' a thiogallate compositions as shown in Table II were made'in the atmosphere Sf yendi refirmg "P HIS about I same manner as the cerium-activated compositions of Examhour at approxfmmely Cr and coolmg to 9 ples 10-1 2 except for the addition of the indicated amount of temperature In nitrogen f p f f the salt of the charge-compensating element to the original phosphors were excited 2y ulu'avlolef and cathodlc rad'anon blend. The observed color and approximate peak emission for eafih P p exhlblted lummosny under both modes of each phosphor under cathode-ray excitation are indicated in excitation. Table 1 sets forth the observed color and approxim TABLE II Cathodoluminescent emission peak Alkaline earth Activator Charge wave. 5 e sulfide compensation length Gags; Product Observed (nanom. (grams) Reagent Grams Reagent Grams Reagent Grams Composition color t Q Example No..

15 4. 71 03.8 1.33 C628; 0. 153 NaOl 0.047 CBJzNBoMGMS Blue-green 465 $0.04. 16 4. 71 SrS 2.20 Same-.. 0.153 ame 0.047 SnbnNaoMGazSn Blue s 80.01- 4.71 BaS 3.12 10 0.153 .d0 0.047 BaamNaomGazSu d0 455 0.04- 4.62 SrS 2.30 -.-do 0.153 znS 0.078 SnEuZnomGai St: .do 4 0 e M 4.71 BaS 0.303 KF 460 0. 093 BaoseKossGazslz Ceom.

mate peak emission for each phosphor under cathode-ray excitation. The emission peaks under ultraviolet excitation (medium pressure mercury vapor lamp-MPMV") were ob served to be at about the same wavelengths.

EXAMPLE [3 Additional samples were prepared of each of the europiumactivated alkaline earth thiogallate phosphor compositions of Examples l-3 in which the europium concentration was varied between 0.0]. and 0.08 gram-atom per mole. The phosphors obtained corresponded in observed color and emission wavelength peak to the phosphors of Examples l-3.

EXAMPLES 21-26 The alkali metal thiogallate compositions shown in Table II] were made by dry-blending gallium sesquisulfide with the car- TABLE I Csthodolumineiscent em sslon Alkaline earth peak sulfide Ac tivator sulfide ave- 3 length (mm- Reagent Grams Reagent Grams Product composition Observed color ometers) Gas 1. 41 E 0. G74 030.0!Gaz84: Ellom 555 SrS 2. 34 E118 0. 074 SI'OJQGB2SIZ Euom 535 B518 3. 32 E118 0. 074 BMJGQISJ: Euom 500 Gas 1. 44 PbS 0. 024 CBoMsGMSs: Pboms 510 SrS 2. 38 PbS 0. 024 Srn .mGazSttPboms 510 Has 3. 37 PbS 0. 024 B80.0||G52S|5Pb0.005 435 Gas 1.33 PbS 0-383 Cao GazSnPbmos 530 SIS 2. 20 PbS 0.383 Stan 08284: Pbom 530 Has 3. l2 lbS 0. 383 BMmGazSazPbms I 1 440, 590 (3:18 1.38 C0 8; 0.153 C%.n( )omGaSa: Blue-groom... 465

00.004. SrS C0 8; 0 153 Sro.u( )omUuzSu Blue... 455 00.04 l2 4. 71 Has 3 25 C028] 0.153 (and )omGazSu .do 455 com Two emission bands.

honate of an alkali metal element and the sulfide of an activator element. The blend was fired in a hydrogen sulfide atmosphere at a temperature of approximately 800 C. for 1 hour and then at 900 C. for an additional hour. The blend was 3. The phosphor composition as defined by claim 2 wherein A is europium and w has the approximate range 0.01 to 0.08 gram-atom per mole.

4. The phosphor composition as defined by claim 3 wherein then allowed to cool to room temperature in a nitrogen at- 5 R is strontium. mosphere. The resulting PhOSphOl'S were excited by ultraviolet 5 The phosphor composition as defined by claim 4 wherein and cathodic radiation and each phosphor exhibited luminosiw i b t 0,02 -at m er mole, ty under both modes of excitation. Table lll sets forth the ob- A P p Composmon consisimg essentially of an served color and peak emission wavelength for each phosphor I kaline earth thiogallate activated by trivalent cerium, the ratio when excited by ultraviolet radiation from an MPMV source. so of alkaline earth sulfide to gallium sesquisulfide in said TABLE III PhOt' luminescent emission Activator $3: G S Alkali metal salt sulfldie length m a na (grams) Reagent Grams Reagent Grams Product composition Observed color omter g) 4.71 K200; 2.75 EuS 0.037 KmaGazSnEuom 4 4.71 NBQCOS E115 0.1 N81 .BUGMSAZEUDM 550 4. 71 K2003 2. 75 PbS 0.024 KLW I HPbOJJOS 592 4, 71 Na co; 2. 035 PbS 0.191 NaLnGBzSuPbam 5 4.71 K2003 2. 74 C825: 0. 038 KimGfizsitCeom 435 4.71 N82CO: 1.865 Ge s; 0.306 N81 .uGmSnCcom 470 EX 27 phosphor composition being about 1:1, said alkaline earth- Additional samples were prepared of the europium-activated sodium and potassium thiogallate phosphor compositions of Examples 21 and 22 in which the europium concentration was varied between 0.00125 and 0.08 gram-atom per mole. The phosphors obtained corresponded in observed color and approximate emission peak to the phosphors of Examples 21 and 22.

EXAMPLE 28 A europium-activated alkali metal thiogallate phosphor composition was prepared by the method of Examples 21-26 except that a combination of alkali metal elements were included in each phosphor. in a first sample 2.51 grams of potassium carbonate and 0.158 gram of sodium carbonate were blended with 4.71 grams of gallium sesquisulfide and 0.037 gram of europium sulfide. The resultant phosphor(K. ,uNa". ;)Ga S.:Euu,m was excited by ultraviolet (M PMV) radiation and found to exhibit blue-green emission having two peaks at about 495 and 518 nanometers. in a second sample, 271 grams of potassium carbonate and 0.02 gram of sodium carbonate were blended with 4.71 grams of gallium sesquisulfide and 0.037 gram of europium sulfide. The resultant phosphor LQKN tI.Q2)() $mGagS-t:Euum exhibited blue-green emission somewhat bluer than the first sample when excited by ultraviolet radiation.

What is claimed is:

1. A phosphor composition selected from the group consisting of R, ,,.Ga S :A,,. and M ,,,.Ga S,:A,,. where R is at least one alkaline earth element selected from the group consisting of calcium, strontium and barium, M is at least one alkali metal element selected from the group consisting of sodium and potassium, A is a divalent activator selected from the group consisting of europium and lead, and w is a value greater than zero selected to produce luminescence when said phosphor composition is exposed to incident radiation.

2. A phosphor composition defined by the formula R, ,,.Ga S ,:A,,. where R is at least one alkaline earth element selected from the group consisting of calcium, strontium and barium, A is a divalent activator selected from the group consisting of europium and lead, and w is a value greater than zero selected to produce luminescence when said phosphor composition is exposed to incident radiation.

being selected from the group consisting of calcium, strontium, and barium.

7. The phosphor composition as defined by claim 6 wherein said alkaline earth is strontium.

8. A phosphor composition consisting essentially of a charge-compensated alkaline earth thiogallate activated by trivalent cerium, the ratio of alkaline earth sulfide to gallium sesquisulfide in said phosphor composition being about 1:1, said alkaline earth being selected from the group consisting of calcium, barium, and strontium.

9. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by sodium.

10. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by zinc.

11. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by potassium.

12. The phosphor as defined by claim 8 wherein said alkaline earth is strontium.

13. A phosphor composition as defined by the formula Sr, ,,.NaGa S,:Ce wherein w has the approximate range 0.01 to 0.12 gram-atom per mole.

14. The phosphor composition as defined by claim 13 wherein w is about 0.04 gram-atom per mole.

15. A phosphor composition as defined by the formula M ,,.GaS,,:A,,. wherein M is at least one alkali metal element selected from the group consisting of sodium and potassium, A is a divalent activator selected from the group consisting of europium and lead, and w is a value greater than zero selected to produce luminescence when said phosphor composition is exposed to incident radiation.

16. The phosphor composition as defined by claim 15 wherein A is europium and w has the approximate range 0.00125 to 0.08 gram-atom per mole.

17. The phosphor composition as defined by claim 16 wherein M is sodium and w is about 0.02 gram-atom per mole.

18. The phosphor composition as defined by claim 16 wherein M is potassium and w is about 0.01 gram-atom per mole.

19. A phosphor composition consisting essentially of an alkali metal thiogallate activated by trivalent cerium, the ratio of alkali metal sulfide to gallium sesquisulfide being about izl, said alkali metal being selected from the group consisting of sodium and potassium. 

2. A phosphor composition defined by the formula R1 wGa2S4:Aw where R is at least one alkaline earth element selected from the group consisting of calcium, strontium and barium, A is a divalent activator selected from the group consisting of europium and lead, and w is a value greater than zero selected to produce luminescence when said phosphor composition is exposed to incident radiation.
 3. The phosphor composition as defined by claim 2 wherein A is europium and w has the approximate range 0.01 to 0.08 gram-atom per mole.
 4. The phosphor composition as defined by claim 3 wherein R is strontium.
 5. The phosphor composition as defined by claim 4 wherein w is about 0.02 gram-atom per mole.
 6. A phosphor composition consisting essentially of an alkaline earth thiogallate activated by trivalent cerium, the ratio of alkaline earth sulfide to gallium sesquisulfide in said phosphor composition being about 1:1, said alkaline earth being selected from the group consisting of calcium, strontium, and barium.
 7. The phosphor composition as defined by claim 6 wherein said alkaline earth is strontium.
 8. A phosphor composition consisting essentially of a charge-compensated alkaline earth thiogallate activated by trivalent cerium, the ratio of alkaline earth sulfide to gallium sesquisulfide in said phosphor composition being about 1:1, said alkaline earth being selected from the group consisting of calcium, barium, and strontium.
 9. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by sodium.
 10. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by zinc.
 11. The phosphor composition as defined by claim 8 wherein said trivalent cerium is charge compensated by potassium.
 12. The phosphor as defined by claim 8 wherein said alkaline earth is strontium.
 13. A phosphor composition as defined by the formula Sr1 2wNawGa2S4:Cew wherein w has the approximate range 0.01 to 0.12 gram-atom per mole.
 14. The phosphor composition as defined by claim 13 wherein w is about 0.04 gram-atom per mole.
 15. A phosphor composition as defined by the formula M2 2wGa2S4: Aw wherein M is at least one alkali metal element selected from the group consisting of sodium and potassium, A is a divalent activator selected from the group consisting of europium and lead, and w is a value greater than zero selected to produce luminescence when said phosphor composition is exposed to incident radiation.
 16. The phosphor composition as defined by claim 15 wherein A is europium and w has the approximate range 0.00125 to 0.08 gram-atom per mole.
 17. The phosphor composition as defined by claim 16 wherein M is sodium and w is about 0.02 gram-atom per mOle.
 18. The phosphor composition as defined by claim 16 wherein M is potassium and w is about 0.01 gram-atom per mole.
 19. A phosphor composition consisting essentially of an alkali metal thiogallate activated by trivalent cerium, the ratio of alkali metal sulfide to gallium sesquisulfide being about 1:1, said alkali metal being selected from the group consisting of sodium and potassium. 